The present invention relates to handoff techniques in cellular telecommunication systems, and, more particularly, to a method and apparatus for mobile assisted handoff between different frequencies in a Code Division Multiple Access (CDMA) cellular system.
Each base station in a cellular system provides a radio interface to mobile stations within a particular area or zone of the system generally referred to as a cell. Each base station is in communication with a mobile switching center which may switch calls from one base station to another base station and connects the base station with the local phone system over landlines or connects the base station with another cellular telephone system. One of the most critical functions associated with mobile telephone use in a cellular system is the transfer of calls from one base station to another as the user travels from one cell to an adjoining cell. This transfer of call control is commonly known as handoff.
As the mobile telephone moves through a cell, the strength of its transmitted signal will vary depending on its distance from the base station. The base station""s range within its cell is designed to drop off rapidly as the coverage range of its cell is reached. If a mobile telephone leaves a cell during ongoing communication and moves into a different cell, it is necessary that a new base station be selected without any disturbance to the communication. In early analog systems, the current base station monitored the strength of the signal emanating from the mobile telephone. As the signal strength lowered to critical levels, a search was conducted of neighboring base stations to determine which base station was receiving the strongest signal from the moving mobile telephone. The switching center would then instruct the mobile telephone through the current base station to change to the necessary parameters of the new base station and handoff occurred.
In more recent Time Division Multiple Access (TDMA) systems where users are each assigned unique time slots on a frequency channel, the mobile stations are designed to measure the strength of certain signals transmitted by surrounding base stations during timeslots when communications are inactive and transmit this information back through the base station to the switching center. This provides the switching center with the information necessary to decide when and to what base station a particular mobile telephone must be switched in order to maintain communication. This technique is called Mobile Assisted Handoff (MAHO).
The most recent type of cellular technology to be developed is Code Division Multiple Access (CDMA). In a CDMA type system multiple users, each using a channel identified by a uniquely assigned digital code, simultaneously communicate with the system while sharing the same wideband frequency spectrum. A CDMA system may be designed so that all base stations share the entire downlink frequency spectrum and all mobile stations share the entire uplink frequency spectrum. In a CDMA system, then, handoff may be necessary between base stations using the same frequencies. This is known as xe2x80x9csoft handoffxe2x80x9d. In soft handoff a mobile station in transition from one base station to another transmits and receives the same signal from both base stations simultaneously. In a CDMA receiver a rake receiver can be used to isolate the signals received at the mobile station from each base station and align them both in time and phase to reinforce one another on the base-to-mobile link. On the reverse link a mobile switching center connected to both base stations resolves which base station is receiving the stronger signal and selects the signal from that base station for the call.
In a CDMA system soft handoff may not always be possible. In these cases hard handoff is used. Hard handoff is handoff in which a temporary disconnection of the communication path (i.e., traffic channel) between a mobile station and a base station takes place. Hard handoffs occur when the mobile station is transferred between disjoint active sets, the CDMA frequency assignment changes, the frame offset changes, or the mobile station is directed from a CDMA traffic channel to an analog voice channel. Hard handoff is distinct from soft handoff wherein communications with a new base station is commenced on the same CDMA frequency assignment before the communications with the old base station are terminated. Hard handoff presents some difficulties in CDMA systems. In general, in a CDMA system a user is assigned all data transmission frames at a CDMA traffic channel sequentially in time. In mobile assisted hard handoff, since the frequencies differ between the systems involved, it would be necessary to use a separate receiver to make handoff measurements at the mobile station, if the mobile assisted handoff were to be noninterfering with ongoing communications.
An object of the present invention is to provide a Code Division Multiple Access (CDMA) cellular system that permits Mobile Assisted Handoff (MAHO) between CDMA base stations that use different carrier frequencies by performing periodic searches of the different carrier frequency base station pilots.
Another object of the present invention is to provide a mobile CDMA cellular system wherein the mobile telephone searches for the base stations on a different carrier frequency without significant degradation in system performance.
The present invention provides a method and system for mobile assisted hard handoff (MAHHO) between base stations using different carrier frequencies in a Code Division Multiple Access (CDMA) cellular system. In the method and apparatus, a mobile station performs periodic handoff measurements on forward link transmissions of handoff candidate CDMA base stations, where the forward link transmissions are transmitted on a carrier frequency that differs from the forward link carrier frequency of the current base station. The handoff measurements are performed during selected transmission frames that define transmission periods on the reverse link by the mobile station. During the measurement time in the selected transmission frame, the mobile station does not receive (puncture) data sent to it on the forward link and does not transmit data on the reverse link.
In an embodiment of the invention, transmission frames including data that are being transmitted at a selected reduced rate are used as the selected transmission frames for mobile station handoff measurements. The position of the measurement periods within the selected transmission frames may be chosen to minimize the amount of data that is not transmitted and lost on the reverse link during the measurement period. The position of the measurement period may also be chosen so that disturbance of other functions of the system, such as, for example, mobile station power control from a base station, is minimized. During the measurement periods the mobile station punctures the received data stream that is transmitted to the mobile station on the forward link.
The handoff measurements may occur periodically. In one alternative that may be used for variable data rate transmissions, such as a voice call using a variable rate coder, a periodically expiring timer causes the next transmission frame that includes data at the selected reduced rate, subsequent to the timer""s expiration, to be chosen as the transmission frame for handoff measurements. In another alternative that may be used for data transmissions having a data role settable independent of the application, the data transmission rate is periodically forced to a selected reduced rate in both the base station and mobile station, and the handoff measurements are periodically performed during the transmission frames that include the data at the selected reduced rate.
In an alternative embodiment of the invention, the selected transmission frames for mobile-station handoff measurements may be chosen periodically but without regard for the data rate of the selected transmission frame. In this embodiment the handoff measurements are performed during the selected transmission frames on the reverse link with no attempt to optimize the position of the measurement period within the selected transmission frame. Data to be transmitted on the reverse link during the selected transmission frame is not transmitted and lost. In this embodiment the mobile station punctures the received data stream that is transmitted to the mobile station on the forward link.